Drain chiles, rinse, and cut a slit down the side of each; gently remove seeds and pith inside.
Stuff chiles with sticks of cheese (about 1/2 inch wide, 1/2 inch thick, and 1 inch shorter than the chile).
Have flour ready in a shallow pan.
Roll each chile in flour to coat all over; gently shake off excess.
Prepare coating (batter) just before you want to fry the chiles.
Coat and fry.
Top with heated sauce and garnish with cheese or onion.